PERLREAPI(1) Perl Programmers Reference Guide PERLREAPI(1)
NAME
perlreapi - perl regular expression plugin interface
DESCRIPTION
As of Perl 5.9.5 there is a new interface for plugging and using other
regular expression engines than the default one.
Each engine is supposed to provide access to a constant structure of
the following format:
typedef struct regexp_engine {
REGEXP* (*comp) (pTHX_ const SV * const pattern, const U32 flags);
I32 (*exec) (pTHX_ REGEXP * const rx, char* stringarg, char* strend,
char* strbeg, I32 minend, SV* screamer,
void* data, U32 flags);
char* (*intuit) (pTHX_ REGEXP * const rx, SV *sv, char *strpos,
char *strend, U32 flags,
struct re_scream_pos_data_s *data);
SV* (*checkstr) (pTHX_ REGEXP * const rx);
void (*free) (pTHX_ REGEXP * const rx);
void (*numbered_buff_FETCH) (pTHX_ REGEXP * const rx, const I32 paren,
SV * const sv);
void (*numbered_buff_STORE) (pTHX_ REGEXP * const rx, const I32 paren,
SV const * const value);
I32 (*numbered_buff_LENGTH) (pTHX_ REGEXP * const rx, const SV * const sv,
const I32 paren);
SV* (*named_buff) (pTHX_ REGEXP * const rx, SV * const key,
SV * const value, U32 flags);
SV* (*named_buff_iter) (pTHX_ REGEXP * const rx, const SV * const lastkey,
const U32 flags);
SV* (*qr_package)(pTHX_ REGEXP * const rx);
#ifdef USE_ITHREADS
void* (*dupe) (pTHX_ REGEXP * const rx, CLONE_PARAMS *param);
#endif
When a regexp is compiled, its "engine" field is then set to point at
the appropriate structure, so that when it needs to be used Perl can
find the right routines to do so.
In order to install a new regexp handler, $^H{regcomp} is set to an
integer which (when casted appropriately) resolves to one of these
structures. When compiling, the "comp" method is executed, and the
resulting regexp structure's engine field is expected to point back at
the same structure.
The pTHX_ symbol in the definition is a macro used by perl under
threading to provide an extra argument to the routine holding a pointer
back to the interpreter that is executing the regexp. So under
threading all routines get an extra argument.
Callbacks
comp
REGEXP* comp(pTHX_ const SV * const pattern, const U32 flags);
Compile the pattern stored in "pattern" using the given "flags" and
return a pointer to a prepared "REGEXP" structure that can perform the
match. See "The REGEXP structure" below for an explanation of the
individual fields in the REGEXP struct.
The "pattern" parameter is the scalar that was used as the pattern.
previous versions of perl would pass two "char*" indicating the start
and end of the stringified pattern, the following snippet can be used
to get the old parameters:
STRLEN plen;
char* exp = SvPV(pattern, plen);
char* xend = exp + plen;
Since any scalar can be passed as a pattern it's possible to implement
an engine that does something with an array (""ook" =~ [ qw/ eek hlagh
/ ]") or with the non-stringified form of a compiled regular expression
(""ook" =~ qr/eek/"). perl's own engine will always stringify
everything using the snippet above but that doesn't mean other engines
have to.
The "flags" parameter is a bitfield which indicates which of the
"msixp" flags the regex was compiled with. It also contains additional
info such as whether "use locale" is in effect.
The "eogc" flags are stripped out before being passed to the comp
routine. The regex engine does not need to know whether any of these
are set as those flags should only affect what perl does with the
pattern and its match variables, not how it gets compiled and executed.
By the time the comp callback is called, some of these flags have
already had effect (noted below where applicable). However most of
their effect occurs after the comp callback has run in routines that
read the "rx->extflags" field which it populates.
In general the flags should be preserved in "rx->extflags" after
compilation, although the regex engine might want to add or delete some
of them to invoke or disable some special behavior in perl. The flags
along with any special behavior they cause are documented below:
The pattern modifiers:
"/m" - RXf_PMf_MULTILINE
If this is in "rx->extflags" it will be passed to "Perl_fbm_instr"
by "pp_split" which will treat the subject string as a multi-line
string.
"/s" - RXf_PMf_SINGLELINE
"/i" - RXf_PMf_FOLD
"/x" - RXf_PMf_EXTENDED
If present on a regex "#" comments will be handled differently by
the tokenizer in some cases.
TODO: Document those cases.
"/p" - RXf_PMf_KEEPCOPY
TODO: Document this
Character set
The character set semantics are determined by an enum that is
contained in this field. This is still experimental and subject to
change, but the current interface returns the rules by use of the
in-line function "get_regex_charset(const U32 flags)". The only
currently documented value returned from it is
REGEX_LOCALE_CHARSET, which is set if "use locale" is in effect. If
present in "rx->extflags", "split" will use the locale dependent
definition of whitespace when RXf_SKIPWHITE or RXf_WHITE is in
effect. ASCII whitespace is defined as per isSPACE, and by the
internal macros "is_utf8_space" under UTF-8, and "isSPACE_LC" under
"use locale".
Additional flags:
RXf_UTF8
Set if the pattern is SvUTF8(), set by Perl_pmruntime.
A regex engine may want to set or disable this flag during
compilation. The perl engine for instance may upgrade non-UTF-8
strings to UTF-8 if the pattern includes constructs such as
"\x{...}" that can only match Unicode values.
RXf_SPLIT
If "split" is invoked as "split ' '" or with no arguments (which
really means "split(' ', $_)", see split), perl will set this flag.
The regex engine can then check for it and set the SKIPWHITE and
WHITE extflags. To do this the perl engine does:
if (flags & RXf_SPLIT && r->prelen == 1 && r->precomp[0] == ' ')
r->extflags |= (RXf_SKIPWHITE|RXf_WHITE);
These flags can be set during compilation to enable optimizations in
the "split" operator.
RXf_SKIPWHITE
If the flag is present in "rx->extflags" "split" will delete
whitespace from the start of the subject string before it's
operated on. What is considered whitespace depends on whether the
subject is a UTF-8 string and whether the "RXf_PMf_LOCALE" flag is
set.
If RXf_WHITE is set in addition to this flag "split" will behave
like "split " "" under the perl engine.
RXf_START_ONLY
Tells the split operator to split the target string on newlines
("\n") without invoking the regex engine.
Perl's engine sets this if the pattern is "/^/" ("plen == 1 && *exp
== '^'"), even under "/^/s", see split. Of course a different regex
engine might want to use the same optimizations with a different
syntax.
RXf_WHITE
Tells the split operator to split the target string on whitespace
without invoking the regex engine. The definition of whitespace
varies depending on whether the target string is a UTF-8 string and
on whether RXf_PMf_LOCALE is set.
Perl's engine sets this flag if the pattern is "\s+".
RXf_NULL
Tells the split operator to split the target string on characters.
The definition of character varies depending on whether the target
string is a UTF-8 string.
Perl's engine sets this flag on empty patterns, this optimization
makes "split //" much faster than it would otherwise be. It's even
faster than "unpack".
exec
I32 exec(pTHX_ REGEXP * const rx,
char *stringarg, char* strend, char* strbeg,
I32 minend, SV* screamer,
void* data, U32 flags);
Execute a regexp.
intuit
char* intuit(pTHX_ REGEXP * const rx,
SV *sv, char *strpos, char *strend,
const U32 flags, struct re_scream_pos_data_s *data);
Find the start position where a regex match should be attempted, or
possibly whether the regex engine should not be run because the pattern
can't match. This is called as appropriate by the core depending on the
values of the extflags member of the regexp structure.
checkstr
SV* checkstr(pTHX_ REGEXP * const rx);
Return a SV containing a string that must appear in the pattern. Used
by "split" for optimising matches.
free
void free(pTHX_ REGEXP * const rx);
Called by perl when it is freeing a regexp pattern so that the engine
can release any resources pointed to by the "pprivate" member of the
regexp structure. This is only responsible for freeing private data;
perl will handle releasing anything else contained in the regexp
structure.
Numbered capture callbacks
Called to get/set the value of "$`", "$'", $& and their named
equivalents, ${^PREMATCH}, ${^POSTMATCH} and $^{MATCH}, as well as the
numbered capture groups ($1, $2, ...).
The "paren" parameter will be "-2" for "$`", "-1" for "$'", 0 for $&, 1
for $1 and so forth.
The names have been chosen by analogy with Tie::Scalar methods names
with an additional LENGTH callback for efficiency. However named
capture variables are currently not tied internally but implemented via
magic.
numbered_buff_FETCH
void numbered_buff_FETCH(pTHX_ REGEXP * const rx, const I32 paren,
SV * const sv);
Fetch a specified numbered capture. "sv" should be set to the scalar to
return, the scalar is passed as an argument rather than being returned
from the function because when it's called perl already has a scalar to
store the value, creating another one would be redundant. The scalar
can be set with "sv_setsv", "sv_setpvn" and friends, see perlapi.
This callback is where perl untaints its own capture variables under
taint mode (see perlsec). See the "Perl_reg_numbered_buff_fetch"
function in regcomp.c for how to untaint capture variables if that's
something you'd like your engine to do as well.
numbered_buff_STORE
void (*numbered_buff_STORE) (pTHX_ REGEXP * const rx, const I32 paren,
SV const * const value);
Set the value of a numbered capture variable. "value" is the scalar
that is to be used as the new value. It's up to the engine to make sure
this is used as the new value (or reject it).
Example:
if ("ook" =~ /(o*)/) {
# 'paren' will be '1' and 'value' will be 'ee'
$1 =~ tr/o/e/;
}
Perl's own engine will croak on any attempt to modify the capture
variables, to do this in another engine use the following callback
(copied from "Perl_reg_numbered_buff_store"):
void
Example_reg_numbered_buff_store(pTHX_ REGEXP * const rx, const I32 paren,
SV const * const value)
{
PERL_UNUSED_ARG(rx);
PERL_UNUSED_ARG(paren);
PERL_UNUSED_ARG(value);
if (!PL_localizing)
Perl_croak(aTHX_ PL_no_modify);
}
Actually perl will not always croak in a statement that looks like it
would modify a numbered capture variable. This is because the STORE
callback will not be called if perl can determine that it doesn't have
to modify the value. This is exactly how tied variables behave in the
same situation:
package CaptureVar;
use base 'Tie::Scalar';
sub TIESCALAR { bless [] }
sub FETCH { undef }
sub STORE { die "This doesn't get called" }
package main;
tie my $sv => "CaptureVar";
$sv =~ y/a/b/;
Because $sv is "undef" when the "y///" operator is applied to it the
transliteration won't actually execute and the program won't "die".
This is different to how 5.8 and earlier versions behaved since the
capture variables were READONLY variables then, now they'll just die
when assigned to in the default engine.
numbered_buff_LENGTH
I32 numbered_buff_LENGTH (pTHX_ REGEXP * const rx, const SV * const sv,
const I32 paren);
Get the "length" of a capture variable. There's a special callback for
this so that perl doesn't have to do a FETCH and run "length" on the
result, since the length is (in perl's case) known from an offset
stored in "rx->offs" this is much more efficient:
I32 s1 = rx->offs[paren].start;
I32 s2 = rx->offs[paren].end;
I32 len = t1 - s1;
This is a little bit more complex in the case of UTF-8, see what
"Perl_reg_numbered_buff_length" does with is_utf8_string_loclen.
Named capture callbacks
Called to get/set the value of "%+" and "%-" as well as by some utility
functions in re.
There are two callbacks, "named_buff" is called in all the cases the
FETCH, STORE, DELETE, CLEAR, EXISTS and SCALAR Tie::Hash callbacks
would be on changes to "%+" and "%-" and "named_buff_iter" in the same
cases as FIRSTKEY and NEXTKEY.
The "flags" parameter can be used to determine which of these
operations the callbacks should respond to, the following flags are
currently defined:
Which Tie::Hash operation is being performed from the Perl level on
"%+" or "%+", if any:
RXapif_FETCH
RXapif_STORE
RXapif_DELETE
RXapif_CLEAR
RXapif_EXISTS
RXapif_SCALAR
RXapif_FIRSTKEY
RXapif_NEXTKEY
Whether "%+" or "%-" is being operated on, if any.
RXapif_ONE /* %+ */
RXapif_ALL /* %- */
Whether this is being called as "re::regname", "re::regnames" or
"re::regnames_count", if any. The first two will be combined with
"RXapif_ONE" or "RXapif_ALL".
RXapif_REGNAME
RXapif_REGNAMES
RXapif_REGNAMES_COUNT
Internally "%+" and "%-" are implemented with a real tied interface via
Tie::Hash::NamedCapture. The methods in that package will call back
into these functions. However the usage of Tie::Hash::NamedCapture for
this purpose might change in future releases. For instance this might
be implemented by magic instead (would need an extension to mgvtbl).
named_buff
SV* (*named_buff) (pTHX_ REGEXP * const rx, SV * const key,
SV * const value, U32 flags);
named_buff_iter
SV* (*named_buff_iter) (pTHX_ REGEXP * const rx, const SV * const lastkey,
const U32 flags);
qr_package
SV* qr_package(pTHX_ REGEXP * const rx);
The package the qr// magic object is blessed into (as seen by "ref
qr//"). It is recommended that engines change this to their package
name for identification regardless of whether they implement methods on
the object.
The package this method returns should also have the internal "Regexp"
package in its @ISA. "qr//->isa("Regexp")" should always be true
regardless of what engine is being used.
Example implementation might be:
SV*
Example_qr_package(pTHX_ REGEXP * const rx)
{
PERL_UNUSED_ARG(rx);
return newSVpvs("re::engine::Example");
}
Any method calls on an object created with "qr//" will be dispatched to
the package as a normal object.
use re::engine::Example;
my $re = qr//;
$re->meth; # dispatched to re::engine::Example::meth()
To retrieve the "REGEXP" object from the scalar in an XS function use
the "SvRX" macro, see "REGEXP Functions" in perlapi.
void meth(SV * rv)
PPCODE:
REGEXP * re = SvRX(sv);
dupe
void* dupe(pTHX_ REGEXP * const rx, CLONE_PARAMS *param);
On threaded builds a regexp may need to be duplicated so that the
pattern can be used by multiple threads. This routine is expected to
handle the duplication of any private data pointed to by the "pprivate"
member of the regexp structure. It will be called with the
preconstructed new regexp structure as an argument, the "pprivate"
member will point at the old private structure, and it is this
routine's responsibility to construct a copy and return a pointer to it
(which perl will then use to overwrite the field as passed to this
routine.)
This allows the engine to dupe its private data but also if necessary
modify the final structure if it really must.
On unthreaded builds this field doesn't exist.
The REGEXP structure
The REGEXP struct is defined in regexp.h. All regex engines must be
able to correctly build such a structure in their "comp" routine.
The REGEXP structure contains all the data that perl needs to be aware
of to properly work with the regular expression. It includes data about
optimisations that perl can use to determine if the regex engine should
really be used, and various other control info that is needed to
properly execute patterns in various contexts such as is the pattern
anchored in some way, or what flags were used during the compile, or
whether the program contains special constructs that perl needs to be
aware of.
In addition it contains two fields that are intended for the private
use of the regex engine that compiled the pattern. These are the
"intflags" and "pprivate" members. "pprivate" is a void pointer to an
arbitrary structure whose use and management is the responsibility of
the compiling engine. perl will never modify either of these values.
typedef struct regexp {
/* what engine created this regexp? */
const struct regexp_engine* engine;
/* what re is this a lightweight copy of? */
struct regexp* mother_re;
/* Information about the match that the perl core uses to manage things */
U32 extflags; /* Flags used both externally and internally */
I32 minlen; /* mininum possible length of string to match */
I32 minlenret; /* mininum possible length of $& */
U32 gofs; /* chars left of pos that we search from */
/* substring data about strings that must appear
in the final match, used for optimisations */
struct reg_substr_data *substrs;
U32 nparens; /* number of capture groups */
/* private engine specific data */
U32 intflags; /* Engine Specific Internal flags */
void *pprivate; /* Data private to the regex engine which
created this object. */
/* Data about the last/current match. These are modified during matching*/
U32 lastparen; /* last open paren matched */
U32 lastcloseparen; /* last close paren matched */
regexp_paren_pair *swap; /* Swap copy of *offs */
regexp_paren_pair *offs; /* Array of offsets for (@-) and (@+) */
char *subbeg; /* saved or original string so \digit works forever. */
SV_SAVED_COPY /* If non-NULL, SV which is COW from original */
I32 sublen; /* Length of string pointed by subbeg */
/* Information about the match that isn't often used */
I32 prelen; /* length of precomp */
const char *precomp; /* pre-compilation regular expression */
char *wrapped; /* wrapped version of the pattern */
I32 wraplen; /* length of wrapped */
I32 seen_evals; /* number of eval groups in the pattern - for security checks */
HV *paren_names; /* Optional hash of paren names */
/* Refcount of this regexp */
I32 refcnt; /* Refcount of this regexp */
} regexp;
The fields are discussed in more detail below:
"engine"
This field points at a regexp_engine structure which contains pointers
to the subroutines that are to be used for performing a match. It is
the compiling routine's responsibility to populate this field before
returning the regexp object.
Internally this is set to "NULL" unless a custom engine is specified in
$^H{regcomp}, perl's own set of callbacks can be accessed in the struct
pointed to by "RE_ENGINE_PTR".
"mother_re"
TODO, see
http://www.mail-archive.com/perl5-changes AT perl.org/msg17328.html
<http://www.mail-archive.com/perl5-changes AT perl.org/msg17328.html>
"extflags"
This will be used by perl to see what flags the regexp was compiled
with, this will normally be set to the value of the flags parameter by
the comp callback. See the comp documentation for valid flags.
"minlen" "minlenret"
The minimum string length required for the pattern to match. This is
used to prune the search space by not bothering to match any closer to
the end of a string than would allow a match. For instance there is no
point in even starting the regex engine if the minlen is 10 but the
string is only 5 characters long. There is no way that the pattern can
match.
"minlenret" is the minimum length of the string that would be found in
$& after a match.
The difference between "minlen" and "minlenret" can be seen in the
following pattern:
/ns(?=\d)/
where the "minlen" would be 3 but "minlenret" would only be 2 as the \d
is required to match but is not actually included in the matched
content. This distinction is particularly important as the substitution
logic uses the "minlenret" to tell whether it can do in-place
substitution which can result in considerable speedup.
"gofs"
Left offset from pos() to start match at.
"substrs"
Substring data about strings that must appear in the final match. This
is currently only used internally by perl's engine for but might be
used in the future for all engines for optimisations.
"nparens", "lastparen", and "lastcloseparen"
These fields are used to keep track of how many paren groups could be
matched in the pattern, which was the last open paren to be entered,
and which was the last close paren to be entered.
"intflags"
The engine's private copy of the flags the pattern was compiled with.
Usually this is the same as "extflags" unless the engine chose to
modify one of them.
"pprivate"
A void* pointing to an engine-defined data structure. The perl engine
uses the "regexp_internal" structure (see "Base Structures" in
perlreguts) but a custom engine should use something else.
"swap"
Unused. Left in for compatibility with perl 5.10.0.
"offs"
A "regexp_paren_pair" structure which defines offsets into the string
being matched which correspond to the $& and $1, $2 etc. captures, the
"regexp_paren_pair" struct is defined as follows:
typedef struct regexp_paren_pair {
I32 start;
I32 end;
} regexp_paren_pair;
If "->offs[num].start" or "->offs[num].end" is "-1" then that capture
group did not match. "->offs[0].start/end" represents $& (or "${^MATCH"
under "//p") and "->offs[paren].end" matches $$paren where $paren = 1>.
"precomp" "prelen"
Used for optimisations. "precomp" holds a copy of the pattern that was
compiled and "prelen" its length. When a new pattern is to be compiled
(such as inside a loop) the internal "regcomp" operator checks whether
the last compiled "REGEXP"'s "precomp" and "prelen" are equivalent to
the new one, and if so uses the old pattern instead of compiling a new
one.
The relevant snippet from "Perl_pp_regcomp":
if (!re || !re->precomp || re->prelen != (I32)len ||
memNE(re->precomp, t, len))
/* Compile a new pattern */
"paren_names"
This is a hash used internally to track named capture groups and their
offsets. The keys are the names of the buffers the values are dualvars,
with the IV slot holding the number of buffers with the given name and
the pv being an embedded array of I32. The values may also be
contained independently in the data array in cases where named
backreferences are used.
"substrs"
Holds information on the longest string that must occur at a fixed
offset from the start of the pattern, and the longest string that must
occur at a floating offset from the start of the pattern. Used to do
Fast-Boyer-Moore searches on the string to find out if its worth using
the regex engine at all, and if so where in the string to search.
"subbeg" "sublen" "saved_copy"
Used during execution phase for managing search and replace patterns.
"wrapped" "wraplen"
Stores the string "qr//" stringifies to. The perl engine for example
stores "(?^:eek)" in the case of "qr/eek/".
When using a custom engine that doesn't support the "(?:)" construct
for inline modifiers, it's probably best to have "qr//" stringify to
the supplied pattern, note that this will create undesired patterns in
cases such as:
my $x = qr/a|b/; # "a|b"
my $y = qr/c/i; # "c"
my $z = qr/$x$y/; # "a|bc"
There's no solution for this problem other than making the custom
engine understand a construct like "(?:)".
"seen_evals"
This stores the number of eval groups in the pattern. This is used for
security purposes when embedding compiled regexes into larger patterns
with "qr//".
"refcnt"
The number of times the structure is referenced. When this falls to 0
the regexp is automatically freed by a call to pregfree. This should be
set to 1 in each engine's "comp" routine.
HISTORY
Originally part of perlreguts.
AUTHORS
Originally written by Yves Orton, expanded by var Arnfjoer` Bjarmason.
LICENSE
Copyright 2006 Yves Orton and 2007 var Arnfjoer` Bjarmason.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.16.3 2013-03-04 PERLREAPI(1)
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